Introduction: The relationship between brain activity and reading performance was examined to test the hypothesis that dyslexia involves a deficit in a specific visual pathway, known as the M pathway, from the eye to the brain. Methods: Functional magnetic resonance imaging (fMRI) was used to measure brain activity in dyslexic and control subjects in conditions designed to preferentially stimulate the M pathway. Results: Dyslexics showed reduced activity compared to controls both in primary visual cortex (V1) and in a secondary cortical visual area (MT+) that is believed to receive a strong M pathway input. Most importantly, significant correlations were found between individual differences in reading rate and brain activity. Conclusions: These results support the hypothesis for an M pathway abnormality in dyslexia and imply a strong relationship between the integrity of the M pathway and reading ability.